Rhinoviruses were tested for sensitivity to human interferon from two sources: human leukosytes induced with Sendai virus and human diploid fibroblasts induced with polyriboinosinic-polyribocytidylic acid. Twenty-five serotypes of rhinovirus were tested against fibroblast interferon in HeLa cells, and five serotypes were evaluated in fibroblast cultures against fibroblast and leukocyte interferons. Sensitivity varied widely in the HeLa cell assay; rhinovirus type 10 was inhibited by approximately 20-40 units of interferon, whereas rhinovirus type 15 was not inhibited by 5,120 units; the significance of this observation is unclear. In contrast, when fibroblast cultures were used in the assay system, five selected serotypes, including rhinovirus types 10 and 15, had similar levels of sensitivity (0.5-5.0 units) to fibroblast or leukocyte interferon.